After “100-year” floods ravaged the region twice in 30 years, planning work is proceeding on an ambitious $130 million effort to re-plumb the Ross Valley by diverting stormwater to huge catch basins in Ross, San Anselmo and Fairfax.

But before work begins on retrofitting Phoenix Lake above Ross and creating water “detention basins” at Memorial and Red Hill parks in San Anselmo and at Lefty Gomez Field and Loma Alta in Fairfax, detailed plans must be drawn following discussions with agencies ranging from the Marin Municipal Water District to local city councils.

“We will be working with all the community partners,” said Tracy J. Clay, county principal civil engineer. “It’s going to be a very collaborative, intensive process.”

“We’re really going to see the physical manifestation of the project over the next 10 years,” noted Supervisor Katie Rice, the Ross Valley representative spearheading the project championed by her former boss, the late Supervisor Hal Brown.

Officials expect that the overall $130 million flood project will be financed largely by state and federal grants. The $15.3 million Phoenix Lake project, for example, has been awarded a $7.6 million state water resources grant intended to cover half the cost.

This fiscal year’s $445 million county budget includes more than $4 million in flood control spending devoted for improvement work in the Ross Valley. Officials say about 35 percent of the tab for all the work envisioned by the overall project involving the lake and other catch basins will be covered by a fee narrowly approved by voters five years ago in a controversial election eventually validated by the state’s highest court after a legal challenge.

The election followed a long history of flooding in the region, including so-called “100-year” floods in 1982 and 2005 that caused damage that ran into the tens of millions of dollars. There is a 1 percent chance of a “100-year” flood occurring in any single year.

Rice and her colleagues on the Board of Supervisors had a bit of good news the other day for taxpayers in Ross Valley, where the unusual flood fee election that required voters to sign ballots split the community and still raises hackles in partisan quarters. The fee, which averages roughly $125 a year, will remain the same this fiscal year, even though the ballot measure allows an annual 3 percent increase.

“Staff recommends that the fee not be increased this year,” Clay said in a report to the county board, which promptly agreed.

The fee levied against 15,000 parcels in Ross Valley Flood Control Zone 9 will raise $2.2 million this year, and, in light of about $1.9 million from other funding sources, covers about half of this year’s $4 million improvement program.

A report endorsed by county supervisors, who must sign off on the methodology used to calculate the fee each year — which is based on estimates of stormwater water runoff each parcel generates — indicates that owners of 9,500 parcels involved will pay $125 or less. The fee schedule indicates 1,534 condo and townhouses will be tapped for $30. Single-family home fees are capped at $180, while commercial industrial parcels will be billed up to $1,141.

While plans for Red Hill Park and other programs may be part of eventual improvements, projects envisioned over the next decade include:

 A $15.3 million retrofit of Phoenix Lake “that will convert the reservoir to a dual use operation” that maintains its water supply function while increasing its capacity roughly 33 percent to serve as a stormwater catch basin.

 A $14.5 million plan to convert acreage at Memorial Park, Lefty Gomez Field and Loma Alta into catch basins that detain high creek flows. Transforming Memorial Park would cost about $6.5 million, and would probably proceed before the $6 million Lefty Gomez or $2 million Loma Alta plans. In any event, earthen berms or other retention walls would be designed to help beautify locales where possible, Clay said.

 A bridge replacement program aimed at nine spans in an effort to increase creek flow capacity. The program would be largely funded by grants.

 Fish passage and habitat enhancement projects.

 Annual creek maintenance programs.

Supervisor Rice said the program is essential. “Retaining flood water in catch basins, or detention basins, during events like the 2005 or 1982 storms, is the only way we can manage flood waters and prevent major flooding in the Ross Valley,” she said. “Detention basins will allow us to hold back a significant amount of water from entering the creek system during the peak of a storm event. They aren’t the entire solution, but they are key components.”

Rice added that the detention basin program will enhance community resources. “The Phoenix Lake project will improve water quality, habitat, trails and public access. … Both the Lefty Gomez and Memorial Park projects would continue to be community playing fields — but aesthetic, environmental, and functional improvements will be incorporated.”

Rice also observed: “The Ross Valley program is ambitious, and it has to be if we are going to have any hope of protecting ourselves from what happened in 2005, and 1982, and so many times before that.”